In the past, a communication system in which a transmission unit (master) and communication devices (slaves) are connected to a transmission path and the respective communication devices communicate with the transmission unit has become popular. As an instance of such a communication system, there has been proposed a system in which the transmission unit periodically monitors states of the respective communication devices and. In this system, upon acknowledging a change in a state of one communication device, the transmission unit sends a signal to another communication device to perform a processing corresponding to the change in the state (see e.g., JP 1180690 B, JP 1195362 B, and JP 1144477 B).
However, in the communication with the above configuration, the communication devices consistently communicate with each other through the transmission unit, and the transmission unit polls the communication devices. Thus, a communication speed is relatively low. For example, this communication system is unsuitable for transmitting information with a relatively large data amount such as an analog quantity. Moreover, as for the aforementioned communication system, when the transmission unit malfunctions, the whole system halts. Hence, the communication system has low system reliability.
Consequently, there has been proposed a communication system designed as a mixture of an existing communication system designed to allow communication devices to communicate with each other through a transmission unit and a communication system designed to allow communication devices to directly communicate with each other via peer-to-peer (P2P) (see JP 2009-225328 A). In this communication system, a transmission path is shared by a first communication device (first communication terminal) communicating with the transmission unit (master) and second communication devices (second communication terminals) directly communicating with each other. Thus, the second communication device can be easily added on the existing communication system.
The first communication device performs first protocol communication using a transmission signal (first protocol signal) which is repeatedly sent from the transmission unit, and the second communication device performs second protocol communication using a superimposed signal (second protocol signal) which is superimposed on the transmission signal. The superimposed signal is superimposed in a predetermined superimposable period included in the transmission signal.
This kind of communication system is exemplified by a system including terminal devices connected to the transmission path via the second communication devices (see FIG. 1). This communication system has two kinds of terminal devices including an upper terminal device 5 and lower terminal devices 61, 62, and 63. In this communication system, a transmission unit 1 is connected to a transmission path 2. The upper terminal device 5 is connected to the transmission path 2 via an upper superimposing apparatus 3 serving as a second communication device. The lower terminal devices 61, 62, and 63 are connected to the transmission path 2 via lower superimposing apparatuses 41, 42, and 43, respectively. The upper terminal device 5 is also connected to the transmission path 2 via another route without passing through the upper superimposing apparatus 3. In the following, when a distinction between the lower superimposing apparatuses 41, 42, and 43 is deemed unnecessary, each of these is referred to as the lower superimposing apparatus 4. Further, when a distinction between the lower terminal devices 61, 62, and 63 is deemed unnecessary, each of these is referred to as the lower terminal device 6.
In the aforementioned communication system, terminal information is transferred between the upper terminal device 5 and the lower terminal device 6 through the second protocol communication established between the upper superimposing apparatus 3 and the lower superimposing apparatus 4. This terminal information is power consumption of an electric appliance measured by the lower terminal device 6, for example. Specifically, when the terminal information is transferred to the upper terminal device 5 from the lower terminal device 6, the lower superimposing apparatus 4 converts the terminal information outputted from the lower terminal device 6 into the superimposed signal and sends the resultant superimposed signal to the transmission path 2, and then the upper superimposing apparatus 3 converts the superimposed signal received from the transmission path 2 into the terminal information and outputs the resultant terminal information to the upper terminal device 5.
According to the communication system using the first protocol communication and the second protocol communication, it is also possible to increase the number of the lower terminal devices 6 by means of adding the lower superimposing apparatus 4.
However, in the aforementioned system, the communication between the upper superimposing apparatus 3 and the lower superimposing apparatus 4 is serial communication using the superimposed signal. When the upper terminal device 5 collects the terminal information from a plurality of the lower terminal devices 6, a time lag of the terminal information is likely to occur between the terminal information of the respective terminal devices.
In more detail, the upper terminal device 5 acquires the terminal information from a plurality of the lower terminal devices 6 one by one. However, the communication between the upper superimposing apparatus 3 and the lower superimposing apparatus 4 is established only in the predetermined superimposable period of the transmission signal. Hence, time necessary to acquire the terminal information per one terminal device is relatively long. Consequently, a timing at which the upper terminal device 5 acquires the terminal information is greatly varied for each of the lower terminal devices 61, 62, and 63. Even when the upper terminal device 5 intends to acquire the terminal information relating to certain time, a relatively large time lag is likely to occur between the terminal information acquired from one lower terminal device 61 and time of the terminal information acquired from another lower terminal device 63.